1. Technical Field
This invention relates to free-space laser communication systems, and more particularly to a free-space laser communication system having at least two communicating transceivers.
2. Background Information
Free-space laser communication systems transmit and receive information by means of a light beam that propagates through space or the atmosphere. Such laser systems gain their principle advantages over radio frequency based broadcast systems by being highly-directional and more difficult to jam. Compared to microwave-based systems, such laser systems usually have greater bandwidth, lower input power, smaller size and less weight. One reason a laser communication source requires less radiated power is because the angular radiated laser beam divergence is much smaller than a microwave beam.
When used for space-based, air-to-air or air-to-ground communications, free-space laser communication systems pose a number of challenging problems. One such problem is the fact that having a smaller beam divergence requires greater accuracy in pointing the laser beam. Microwave beam divergence is typically on the order of milliradians whereas laser beam divergence is generally less than 0.1 milliradians. This characteristic requires pointing accuracies for laser beams on the order of 1 to 10 microradians.
Accordingly, the first step in establishing communication for two laser communication terminals is for each terminal to acquire and track the other terminal. Typically such laser communication terminals will include a tracking beacon, a beacon receiver, and a communication transceiver, which are referred to as optical apertures. The transceiver optical aperture generally is mounted on a gimbaled platform having at least two orthogonal axes of freedom and which has been stabilized against base motion. The optical apertures are optically aligned with each other to what is called the system boresight.
The beacon laser of each terminal radiates a signal with much larger beam divergence than the signal of a separate communication laser, thus providing a source to acquire and track. This source must be viewable with good signal strength in the presence of other light, such as sky light, sunlight, starlight, moon light, or light reflected from the earth and other objects.
The acquisition and tracking system of each of a pair of laser communication terminals must be able to initially point the transmitting and receiving apertures of each terminal as close as possible to the direction of the other terminal.
FIG. 1 is a stylized diagram showing two vehicles communicating by means of a free-space laser communication system. A first vehicle 1 (e.g., an airplane) has a gimbal-mounted host laser communication terminal 2 mounted so as to be able to xe2x80x9cseexe2x80x9d a similarly mounted target laser communication terminal 3 on a second vehicle 4 (e.g., an airplane). The host terminal 2 includes a pointing system, a coarse acquisition and tracking system for generating a large field of view (FOV) xe2x80x9cfootprintxe2x80x9d 5 to illuminate the target terminal 3, and a fine acquisition and tracking system for generating a smaller xe2x80x9cfootprintxe2x80x9d 6 to more precisely illuminate the target.
Certain aspects of the system shown in FIG. 1 are disclosed in U.S. Pat. No. 5,710,652, entitled xe2x80x9cLaser Communication Transceiver and Systemxe2x80x9d and U.S. Pat. No. 5,801,866, entitled xe2x80x9cLaser Communication Devicexe2x80x9d and U.S. patent application Ser. No. 08/221,527, filed Apr. 1, 1994, entitled xe2x80x9cPoint-to-Point Laser Communication Devicexe2x80x9d (now U.S. Pat. No. 5,754,323); [Ser. No. 08/199,115, filed Feb. 22, 1994, entitled xe2x80x9cLaser Communication Transceiver and Systemxe2x80x9d;] and Ser. No. 07/935,899, filed Aug. 27, 1992, entitled xe2x80x9cVoigt Filterxe2x80x9d (now U.S. Pat. No. 5,731,585). Each of the above references is incorporated herein by reference.
FIG. 2 is a stylized diagram showing the angular field of view (FOV) 7 for the optical apertures of the target laser communication terminal 3 on the second vehicle 4 and the angular FOV 5 of the beacon beam from the transmitting host laser communication terminal 2. Each laser communication terminal must be able to initially point its transmitting and receiving optical apertures as close as possible to the direction of the opposite, target terminal. The beacon beam 5 from the host terminal 2 of the first vehicle 1 must provide a large footprint 5 at the receiving target terminal 3 to give the greatest probability of illuminating the target terminal 3. The target terminal sensor should have a large angular field of view 7 to improve the probability of seeing the host terminal""s beacon beam on the first xe2x80x9clookxe2x80x9d. This will reduce the amount of searching time and the uncertainty in establishing the communication link. However, if the beacon beam divergence is made too large, the intensity of the received beacon signal may be so low that the tracking signal caused by the received beacon signal is obscured by system electronic noise and other illuminating light sources.
The platform on which each terminal is mounted must provide a means to stabilize the pointing of the transmitting and receiving optical apertures against angular disturbances of the base mount. The base mount could include a vehicle, such as an aircraft or space platform, which has a significant amount of angular motion which would cause pointing errors for the optical apertures. The ideal method of stabilization would be a totally frictionless mount which has freedom to rotate in two orthogonal directions. If a frictionless mount were possible, then system inertia would cause each terminal to stay pointed in the same direction in the presence of angular disturbances to the base mount. In practice, friction couples base motion to the optical apertures of a terminal, causing angular motion. Such angular motion should be removed with a servo system which both senses and provides opposing torques to stabilize against the base motion.
The frequency of base motion disturbances can vary widely. Aircraft often have base motion disturbances at propulsion system frequencies or some multiple of these frequencies. A military tank would have disturbances at the frequencies of the engine rotation. These base motion disturbances can cause large pointing errors in a laser communication system. The terminal servo system must sense these disturbances and stabilize the base mount of the terminal. In general, the servo system must have a frequency response sufficiently high to compensate for the highest frequency components of base motion that contribute to producing pointing error.
The tracking system also must be able to sense angular motion of a terminal and provide pointing correction. The transmitter should be pointed with greater precision than the receiver of a laser terminal, since the beam divergence angle of the transmitter is much smaller than the receiver""s acceptance angle. The precision of pointing the communication laser beam is preferably a fraction of its beam divergence angle that is sufficiently small so that the received power will vary at the receiver as the beam jitters due to the exponential decay in the beam intensity from its central maximum to its edge. The system is usually configured to maintain the beam power density equal to or above one half of its maximum power density. This ensures that the received communication signal is at least one half of the maximum transmitted power density that could be received during normal motion of the transceivers and in the presence of base angular vibration. In general, the angular tracking rates are small when terminals are separated by large distances. However, satellite to satellite tracking rates can be quite large for some systems, and ground to air tracking rates can be large if smaller distances are involved.
To provide tracking, a beacon receiver of terminal will image an incoming beacon beam onto a pixel-imaging device of what is commonly referred to as a xe2x80x9ccentroiderxe2x80x9d. The centroider provides an error signal which is directly proportional to the angular difference between the boresight of the receiving terminal and the line of sight to the opposite transceiver. The error signal is amplified and filtered and then applied to gimbal drive motors or actuators to position the optical apertures of the receiving terminal such that the error signal is reduced to a minimum. For a free-space laser communication system with one or both terminals mounted on moving platforms to achieve a long range up to or greater than 500 km and a high data rate up to or greater than 1 GBPS, the communication laser beam must have a very small beam divergence angle in order to achieve the power density needed at the receiving terminal. This requires great precision in pointing because any errors greater than a desired beam divergence (e.g., the half angle beam divergence) would cause the footprint of the transmitted laser beam to miss the receiving aperture.
Some conventional designs use a common aperture configuration to achieve the above precision pointing. The transmitted communication laser beam is transmitted through the same aperture for receiving the beacon signal. Since the received beacon beam signal is used for determining the pointing direction, both apertures may be configured to have the same optical layout by using the same optical elements. This ensures that the communication laser beam is pointed in exactly the same direction as the received beacon signal. Such common aperture configuration also significantly reduces or minimizes any misalignment caused by temperature expansion, mechanical vibration or long-term drift of the mechanical misalignment.
One limitation of the common aperture configuration is the high background noise due to sharing of the same optical elements by a high power light source, the communication laser beam, and a sensitive receiver. Reflection or scattering of light from the optical surfaces caused by, for example, surface irregularities, optical coatings, optical element voids/inclusions, or accumulated particulate matter on the surfaces, can increase the background noise of the receiver. In a typical system, the sensitivity of the receiver is on the order of nanowatts while the transmitted power is many orders of magnitude greater (e.g., a fraction of a watt which is approximately 108 greater than the receiver sensitivity). Such high background noise essentially decreases the allowable maximum separation between the two transceivers, thus undesirably reducing the communication range of the system.
Therefore, a need exists for improving accuracy of tracking and pointing in presence of the base motion and for increasing the transceiver range for such a free-space laser communication system. One aspect of the present invention is to provide a system to meet such need.
The invention is embedded in a free-space laser communication system having a fine tracking and acquisition system with six axes of movement. The tracking and acquisition system includes a low inertia steerable mirror having two axes of movement which points the communication laser transmission optical apertures separately from the optical apertures for the beacon laser, coarse tracker, and communication receiver of the system. A fine tracking receiver centroider and the communication lasers share a common steerable mirror to substantially reduce or eliminate any alignment error that might arise from use of a separate deflecting element for each. The separate apertures for the communication laser the communication receiver significantly reduces the amount of background light received by the communication receiver from its communication transmitter. The system is configured so that the steering mirror, the received beacon and the transmitted lasers use different regions of the steering mirror for reflection to prevent light from the transmitted laser from entering the fine tracking receiver. The fine tracking and acquisition system is preferably mounted on a gimbal having two axes of movement, and the gimbal in turn is preferably mounted in a housing having two axes of movement.
One embodiment of a free-space laser communication system having six axes of movement, includes: a housing having two axes of movement; a gimbal mounted within the housing, the gimbal having two axes of movement; a free-space laser communication transceiver mounted on the gimbal. The transceiver includes a centroiding system for determining an aiming point for the free-space laser communication system; a steering mirror for directing an incoming beacon beam to the centroiding system, the steering mirror having two axes of movement; a feedback control for aiming the steering mirror at the aiming point determined by the centroiding system; and at least one communication laser beam directed at the steering mirror and thereby directed at the aiming point.
The free-space laser communication system may include a fine acquisition and tracking system which comprises: a centroiding system for determining an aiming point for the free-space laser communication system; a steering mirror for directing an incoming beacon beam to the centroiding system; a feedback control for aiming the steering mirror at the aiming point determined by the centroiding system; at least one communication laser beam directed at the steering mirror and thereby directed at the aiming point.
One implementation of the fine acquisition and tracking system has a control bandwidth in excess of 400 Hz, which is 6 to 8 times the bandwidth that can be achieved with only a mechanical gimbal aperture stabilization system. The invention addresses the high bandwidth stabilization and tracking requirements of a free-space laser communication system capable of long range (e.g., 500 km) and high data rates (e.g.,  greater than 1 GBPS) and with one or both laser terminals mounted on moving platforms.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.